Buzzards Bay National Estuary Program

Home    BBNEP Info    Town Information    Pollution    Living Resources    Watershed Info    Enjoy Buzzards Bay    Technical Info    Special Topics

Home  >   Living Resources  >   Eelgrass  >   Eelgrass Historical changes

Historical Changes of Eelgrass in Buzzards Bay

Sub-pages:   Codium Explosion off Wareham |  West Falmouth |  West Island | 

Related pages: Physical Disturbance of Eelgrass  |  Eelgrass GIS Data

Historical Changes in Eelgrass Abundance

by Dr. Joe Costa

For the 2003 State of Buzzards Bay Conference, and in the Coalition for Buzzards Bay's 2007 and the planned 2012 State of the Bay reports, the Buzzards Bay NEP estimated historical changes of eelgrass cover in Buzzards Bay from the 1930s to the present day based on aerial photographs, as well as written and anecdotal information contained in earlier reports. The Buzzards Bay NEP also included a more speculative assessment of what eelgrass abundance might have been in pre-colonial time when Buzzards Bay was pristine. Our analysis took into account the 1931-32 the wasting disease that destroyed at least 90% of eelgrass beds in the North Atlantic, but with Buzzards Bay probably losing closer to 99% of its eelgrass cover, as well as more recent trends caused by changes in water quality and clarity.

For the 2012 Buzzards Bay state of the Bay report, we have included new survey data conducted by DEP for selected embayments in 2006 and 2010. We also analyzed for the first time, additional archival photographs from the 1950s, 1960s, and 1970s, and newer aerial photography from 2005-2010. This allowed us to define with more prec sion the historical recovery of eelgrass in Buzzards Bay after the wasting disease, detailed trends for specific embayments and outer coast areas, and new declines evident during the past 20 years. This new analysis helped us refine average abundance of eelgrass for each decade between the 1950s and today. In the sections below we describe important information and data sources used, and assumptions included in the analysis.

Buttermilk Bay Eelgrass Cover in October 1951 versus September 1994.

Costa 1988 EPA Report in Thesis Data

With funding from the Buzzards Bay Program in 1985, Costa mapped current and historical distribution of eelgrass in Buzzards Bay based on field surveys between 1983 and 1987, historical aerial photographs between 1938 and 1983, and sediment cores (Costa 1988a, 1988b). A description of the methodologies used and the limitations of using older aerial surveys are found on the eelgrass GIS data page. Important considerations when evaluating old aerial photographs include whether or not the photographs were taken too early in the growing season to enable easy visualization of annual eelgrass beds, whether or not a recent hurricane or winter icing affected eelgrass abundance in shallow waters, and whether or not water transparency is adequate to enable visualization of the lower edge of the beds.

The 1988 eelgrass reports included historical changes of eelgrass abundance for selected sites. More recently, the Buzzards Bay NEP has been scanning and registering original source photographs and acquiring others, and using current GIS software and methods, creating new GIS coverages of eelgrass change that will be posted on this website.

1931-1932 wasting disease

A "wasting disease" destroyed virtually all eelgrass in Buzzards Bay (and elsewhere in North America) during 1931-32. The disease appears to have been caused by a virulent cellular slime mold that might have been introduced to the Atlantic or facilitated by exception warm weather during those summers. All documentation suggests that eelgrass populations equaled or exceeded present-day abundance prior to this catastrophe. Photographs taken 6 to 10 years after the disease show that eelgrass covered less than 10% of the present-day habitat area in Buzzards Bay, and many areas were not recolonized for decades. Most areas of Buzzards Bay had not fully recovered from the wasting disease until the mid 1960s.

The process of recolonization was similar in many areas: new beds initially appeared on bare substrates, beds expanded, additional new beds appeared, and some beds were removed by disturbance. In this way eelgrass population saturated small areas (1-10 ha) 5 to 15 years after initial colonization. Rates of eelgrass colonization over larger regions (100's of ha) depended on distance from refuge populations and heterogeneities of the environment. The greatest rates of eelgrass expansion occurred during the 1950's and 1960's. Most available substrate was saturated by the 19801s, but eelgrass continued to expand in some areas, or show new pollution-related declines in others.

DEP Eelgrass Surveys surveys

DEP first conducted aerial surveys of eelgrass in Buzzards Bay in 1996 (a recommendation in the Buzzards Bay 1991 Comprehensive Conservation and Management Plan). One problem with the aerial survey was that the photographs were taken June 1, before most functional annual eelgrass beds (that is, those growing from seed) were established. While DEP did conduct field survey during the following summer, such surveys only corroborate the presence or absence of eelgrass in a relatively few number of points. This made it somewhat difficult to compare some areas in the DEP survey, to the 1988 Buzzards Bay reports, which were largely based on a October 1981 coastal aerial survey where annual and perennial eelgrass beds show the greatest contrast with background sediments because of high leaf and epiphyte biomass. This 1981 aerial survey was also undertaken on a date and time with both a low tide and exceptional water clarity so that deep beds were also easily discerned. Because of these differences, it was difficult to determine whether the apparent absence of eelgrass in the 1996 survey was based on pollution, disease, or icing losses, or limitations of the aerial survey.

It was not until, DEP repeated its eelgrass aerial and field survey in 2001 and 2002, and when we began reviewing other sets of aerial photographs that it became apparent that some eelgrass losses documented in 1996 in certain parts of Buzzards Bay (like in the Wareham River and off Wareham's Great Neck) were real and worsening (read this Codium versus Eelgrass off Great Neck article). The 2001 aerial survey was flown somewhat later (mid-June), but this time period still lead to underestimates of shallow annual bed cover. This survey also had only average water transparency, so some deeper beds were not mapped.

In 2003, DEP mapped eelgrass at a large number of sites in the Commonwealth using a 1951 state highway department aerial survey photograph set. These photographs were taken from a low altitude during the fall, and generally show good water transparency. Because many areas still had not recovered from the wasting disease, this eelgrass cover should not be used to determine the possible maximum of extent eelgrass in pollution and nitrogen TMDL studies, especially if aerial surveys from the 1960s, 70s, or 80s show a greater extent of eelgrass cover. DEP has not yet made the 1951 eelgrass survey coverage available through the MassGIS website.

Development of New Coverages.

We scanned in detail for the first time photos from Costa thesis aerial archive, as well as downloaded new and archival photographs from various. Using ArcGIS 9.3, images were easily imported and georeferenced generally to within less than 10 ft of current MassGIS orthobasemaps. Images were typically enhanced within ARCGIS by stteching raster histograms and changing brightness and contrast. This ability to enhance aerial images enabled visulation of deeper beds and deep edge boundary locations that were imposible to evaluate using tracing and transfer techniques common a decade ago. Bed boundaries were heads up digitized in the GIS software with the imagery magnified to capture eelgrass habitat boundaries and bare patches within beds, often with point spacings less than 10 meters. This approach enabled us to create eelgrass habitat maps with a high degree of prcesion to the imported base maps. Because this fineness of detail was not consistent with some earlier DEP eelgrass coverages, in a few embayments the DEP eelgrass bed boundaries were reevaluated to better capture bed patchiness to ensure better better consistency with new coverages developed.

Hypothetical Eelgrass Distribution During the Colonial Period

For the Coalition State of the Bay reports we estimated eelgrass cover circa 1600. This is of course a speculative exercise, but it is worthwhile to imagine the potential distribution of eelgrass in Buzzards Bay without human impacts such as nitrogen loading, increased water turbidity associated with urban runoff, resuspension of sediments from boat traffic, and other human disturbances.

Eelgrass distribution is largely dependent upon water transparency. This is evident in the clear waters of offshore areas of Cape Cod and Islands, where eelgrass often grows between 20 and 30 feet MLW, and deeper depths of eelgrass beds have been recorded by divers. In less polluted and better flushed harbors and coves, eelgrass beds can still be found to depths between eight and twelve feet. In contrast, in most polluted embayments, eelgrass, if present at all, may only grow to depths of 6 feet MLW or often much less.

Using the depth of growth of eelgrass in clean waters, we can speculate what eelgrass abundance in Buzzards Bay may have once been, and a hypothetical historical eelgrass habitat in Buzzards Bay can be postulated. Such hypotheses can even be tested because Costa (1988, 1989) and others have shown that the remains of eelgrass seed coats (tests) are a valuable biostratigraphical marker that remain in the sediments of many bays and harbors which can be documented in sediment cores. For example, no eelgrass has been found in inner Apponagansett Bay (Dartmouth) for decades, but sediment cores show eelgrass was abundant for centuries (Costa, 1988a).

For the estimation of eelgrass cover in 1600, the following two assumptions were made. For upper Buzzards Bay (roughly a line drawn from North Falmouth to Mattapoisett), we assumed eelgrass grew to 12 ft. MLW. For lower half Buyzzards Bay, we assumed eelgrass grew down to 20 feet MLW in lower Buzzards Bay.

Hypothetical eelgrass cover circa 1600. From Costa 2003 State of Buzzards Bay presentation.

1980s vs. 1990s Changes in Distribution

Comparison between the Costa 1980s and DEP's 1996 mapped distribution of eelgrass.

Comparison between the Costa 1980s and DEP's 1996 showing decline in eelgrass cover in the Wareham area

Comparison between the Costa 1980s and DEP's 1996 showing decline in eelgrass cover in the West Falmouth Harbor area.

Comparison between the Costa 1980s and DEP's 1996 showing increase in eelgrass cover around Clarks Point New Bedford area.

Click to enlarge (200 kb jpg file).

Comparison between the Costa 1980s, DEP's 1996, and the hypothetical precolonial distribution of eelgrass around Marion, MA.

Click to enlarge (1.8 MB jpg file).

Comparison between the Costa 1980s, DEP's 1996, and the hypothetical precolonial distribution in all of upper Buzzards Bay.

Historical Summary presented at 2003 State of the Bay conference and new DEP data for 2001

Historical Summary of eelgrass in Buzzards Bay from Costa 2003 State of Buzzards Bay presentation and 2006 state of the bay report updates.

Eelgrass Links and Reports on Historical Changes

Costa, J. E. 1988a. Eelgrass in Buzzards Bay: Distribution, Production, and Historical Changes in Abundance. EPA 503/4/88-002 204 pp (5.4 MB)
This report contains much the same information as the thesis work, except that it contains additional detailed embayment specific site histories.

Costa, J. E. 1988b. Distribution, production, and historical changes in abundance of eelgrass (Zostera marina L. ) in Southeastern MA. Ph. D. Thesis, Boston University, 395 pp.
(7.5 MB pdf)


DEP eelgrass interactive maps.

GIS shape files from MassGIS.

A May 12, 2004 letter to the Wareham Marine Resources Commission about the distribution of eelgrass in Onset Bay in response to a request for information.


PDF fact sheet on Massachusetts DEP eelgrass mapping program, and the Wetland Conservancy Program

MassGIS - DEP Eelgrass Data layer Description and download

NOAA Project: Loss of eelgrass at Cape Pogue Bay on Martha's Vineyard, Massachusetts, 1988-1994

---